It's a truly marvellous point in a band's career when they decide it's time their listeners become party to their "experimental" side. Marvellous for them, that is. For them, the experimental stuff is what makes their hearts beat that bit faster, the bit that makes them gaze in wonder at each other across a rehearsal room, their eyes gleaming, their foreheads shining with perspiration, as they make a racket that's entirely unlike the racket everyone knows them for. Unfortunately, for the rest of us, it's quite often the point where we realise that, actually, we're quite busy with all those other bands we like and maybe we sort of need a break from each other for a while. So who's excited by the news that Peter Bjorn and John's new album, Living Thing, displays their more "experimental" side?

It has songs that touch on Brazilian batucada and Afropop. It has at least two songs that appear to be about going mad.

For the first seven of their 10 years together, Stockholm's Peter (Morén: vocals, guitar and harmonica), Björn (Yttling: bass guitar, keyboards and vocals) and John (Eriksson: drums, percussion and vocals) wrote songs that a tiny number of people loved. Consequently the band spent a long time sharing bedrooms - and even beds - on tour and selling not many records. Their first two albums, 2002's Peter Bjorn and John and 2005's Falling Out were perfectly pleasant, dreamily noisy, indie-rock-flavoured affairs - the band came together through a shared love of American pop-stylists the Lilys - that were heard by almost no one outside of the band.

Then, in 2006, they wrote a song called Young Folks and everything changed. A beautifully simple song consisting of a drum track, a bassline, a boy/girl vocal, a splash of guitar and a hint of bongo, capped with milkman-friendly whistling, it is the sort of record that makes you feel glad to be alive. The track roared off their third album, Writer's Block, and propelled them to level of success they barely knew existed. A year after they recorded it, Young Folks ended up on a Kanye West mixtape.

"I remember the first day we played it in a small disco in Stockholm," says Morén in a basement bar near their record company's office. "It felt like every person in the club came up to ask what it was. Two years on, I can look back and see that the song was an antidote to something in the air. It belongs to everyone now, they get their own feelings out of it - it's not ours any more.

"When you write a song like that, you have to let it leave home," says Eriksson. "Let it get drunk. Maybe screw around a bit ..."

"It was a huge surprise to me," says Yttling. "But Swedish people are naturally pessimistic. I moved to New York for three months and I got to understand the record more there by watching people's reactions to it."

"It was quite a shock after five years of absolutely no money," says Morén, smiling now. "We come from a place of complete obscurity, so suddenly touring all over the world and having this huge record was a shock to the body and the mind. Having a record like Young Folks behind you gives you the feeling it's OK to do this. I always thought I was a great songwriter, but it never seemed to mean much. But now it's great - and I like it!"

But the question is, do they really like it? Peter Bjorn and John are not as other bands. They proudly claim, "We're not a real rock band", and while there is nothing wrong with that at all - there are far too many rock bands in the world and the vast majority of them are rubbish - it does leave you wondering how much they relish the experience of being in a band.

"We're singers and songwriters and musicians and producers," says Morén. "We're friends, it's always been different for us. We don't feel like a real rock band, that's why we picked the name. We didn't want a name like Pet Shop Boys or Datsuns, something that told you what we'd be like."

But PBJ can't help but give the impression they'd have been perfectly happy if no one had taken any notice of Young Folks in the first place. This is, after all, a band who followed up their breakthrough album with a sombre collection of instrumentals devoted to lonely Scandinavian childhoods. You can almost hear those planning meetings at their label, can't you?

"There were some interesting silences, yes," says Eriksson.

"We wanted a change of pace after two years of touring," says Morén. "We wanted to show people that we weren't this twee pop group. We wanted people to know we're a weird band."

So Seaside Rock was, in fact, a deliberate two fingers to your audience?

"There was some finger-showing," nods Morén, "definitely."

"We can make the occasional hit," says Eriksson, "but people should prepare to be surprised. We will take weird turns, keep people guessing."

From the outside it would appear that career management isn't that high on your list of priorities.

"That's exactly right," Morén says. "We don't have any at all. Maybe we should. Maybe some day we could talk about what we actually want. But that's not how it works with this band. We do everything by chance."

That is a refreshing attitude - groups who've worked the whole thing out in advance tend toward the awful. However, when someone has written one of the truly great songs of the last decade it's a bit galling if they decide to spend the next few years pretending they didn't. There is a theory that although musicians all desire attention, they get terrified of suddenly being "uncool" when the public notices them. Hence their desire to retreat into "experimental" work and instrumental albums. There seems to be a strong streak of that in Peter Bjorn and John. "It is hard when people have their own ideas about what you do," says Eriksson. "That's true."

"What's hard to decide is what to do with the band," says Morén. "We want to stick around and keep an audience, but we don't want to become too big. We want to make a living, but we don't want to play stadiums. The choices are hard. Not that we're about to become U2, anyway."

"I like tennis," says Eriksson, "but I don't want to win Wimbledon. We can't stand restrictions. We want to do instrumental albums and solo projects and not have people grumping about it."

"What we have now is the best level," says Morén. "We'd like to keep this going for another five or six years. That would be amazing."

That is not a phrase you hear from bands terribly often. In a way it's refreshing, but in another it seems a shame: they could become a truly great band, but they're not sure that, as intelligent, witty men in their mid-30s, they could knuckle down and deal with the bullshit long enough to make it happen.

Ytlling is the most serious of the three yet even he's not entirely serious. He describes the band as "three male Lily Allens", but admits their name makes them sound like "a team of lawyers". But while Morén and Eriksson seem happy to have racked up a single Young Folks, Yttling wants more. "The truth is, you have to work fucking hard every single day," he says. "You can't relax with only one Young Folks, you need at least two, maybe three, to keep it all moving."

Yttling is also the most restless of the three - his production work has become sought-after since Young Folks, and he's worked with Primal Scream and CSS, among others. "I couldn't fill my day only doing Peter Bjorn and John stuff," he says. "There just wouldn't be enough music making and I don't like too much time on my hands."

Did Kanye offer you any advice, I ask. Morén pauses for a long time before saying, "No. But he did say if you have ginger in your food, it helps you stay well."

"We should give him advice," says Eriksson. "He needs to buy less shoes and save the planet. He has 450 pairs of shoes! I think he should keep that quiet - it sounds like an illness. I mean, I have 450 pairs of long-johns, but I've never told anyone about it before now."

Would you like to be left alone for a bit now? "Maybe," says Morén. "We want to make an album this year and next year, then take a rest. Then we'll really see what's happening."